Honeybees, Apis mellifera, are required for the effective pollination of crops and are therefore critical to world agriculture. Honeybees also produce economically important products, including honey and bees wax. The health and vigor of honeybee colonies are threatened by numerous parasites and pathogens, including viruses, bacteria, protozoa, and mites, each with characteristic modes of transmission.
In general, transmission of viruses can occur via two pathways: horizontal and vertical transmission. In horizontal transmission, viruses are transmitted among individuals of the same generation, while vertical transmission occurs from adults to their offspring. Transmission can occur through multiple routes in social organisms (for a detailed review see Chen Y P, et al (2006) Appl Environ Microbiol. 72(1):606-11). Recently, horizontal transmission of honeybee viruses has been documented in bee colonies, for example, transmission of deformed wing virus (DWV) and Kashmir Bee Virus (KBV) by the parasitic mite Varroa destructor, as well as some evidence of virus in honeybee eggs and young larvae, life stages not parasitized by Varroa mites.
Varroa (Varroa destructor) mites are the number one parasite of managed honey bees (Apis mellifera) and the biggest global threat to commercial beekeeping (Rosenkranz et al. 2010). Varroa mites parasitize pupae and adult bees and reproduce in the pupal brood cells. The mites use their mouths to puncture the exoskeleton and feed on the bee's hemolymph. These wound sites in the exoskeleton harbor bacterial infections, such as Melissococcus pluton, which causes European foulbrood. In addition, to their parasitic effects, Varroa mites are suspected of acting as vectors for a number of honey bee pathogens, including deformed wing virus (DWV), Kashmir bee virus (KBV), acute bee paralysis virus (ABPV) and black queen cell virus (BQCV), and may weaken the immune systems of their hosts, leaving them vulnerable to infections. Some bee viruses are known to replicate in the mite, thus dramatically increasing the viral load. If left untreated Varroa infestations typically result in colony-level mortality.
Currently, beekeepers use a plethora of methods to control Varroa levels that include various chemical miticides, most of which have lost efficacy and are toxic and/or leave residues in wax and honey. Other methods include application of oxalic or formic acid, monoterpenes (thymol) and a variety of other management practices, with highly variable outcomes, including toxicity to the treated colonies. Breeding of bees for resistance to Varroa, such as selection for Hygienic behavior which results in the removal of infested brood, has provided a limited practical success.
Current methods of treating Varroa infestations are proving to be ineffective as the mites develop resistance to existing miticides. In addition, the use of such miticides may introduce injurious chemicals into honey that is intended for human consumption.